In the image forming apparatus using electrophotographic process such as a photocopier, printer, facsimile and multi-functional peripheral provided with the functions thereof; a latent image corresponding to a manuscript is formed on a photoreceptor, and toner is applied to this latent image, which is thereby developed. This developed toner image is transferred onto the sheet of paper. After that, the toner image transferred on the sheet is fixed in position and is then ejected.
One of the aforementioned fixing devices for fixing the toner image is a fixing device using a heated roller fixing method, wherein the sheet with the toner image transferred thereon is sandwiched and conveyed by the nip section formed of a fixing roller (fixing member) incorporating a halogen heater and others as a heating means, and a pressure roller (pressure member) for applying pressure to the fixing roller. The sheet is heated and pressed while it is conveyed. This fixing device is used over an extensive range because of simple configuration.
Another of the aforementioned fixing devices for fixing the toner image is a fixing device using a belt fixing method, wherein the endless fixing belt (fixing member) is applied to a heating roller incorporating a halogen heater and others, and a fixing roller; and a pressure roller for applying pressure to the fixing roller through a fixing belt is provided. While the sheet with a toner image transferred thereon is sandwiched and conveyed by the nip section formed of a fixing belt and pressure roller, the sheet is heated and pressed during this time. Such a fixing device has a small heat capacity of the fixing belt This reduces the warming up time, and provides an advantage of energy conservation.
In this case, the toner of the toner image of the sheet is heated while the sheet is passed through the nip section. Since the toner has an adhesive strength, the sheet passing through the nip section may stick to the surface of the fixing roller and fixing belt, and may wind the roller without being separated, with the result that a paper jam may occur. The separability will be reduced especially when the coated sheet of a smaller basis weight for printing (thin paper) is used.
In the meantime, if the fixing roller size is increased in order to ensure the sufficient nip width to cope with an increasing speed of the image forming apparatus, the roller curvature at the outlet of the fixing nip will be reduced, with the result that the separation performance will be reduced.
A separation member is used to cope with the reduction in the separation performance. This means uses a blower means and blower duct to blow air to the outlet side of the nip section for the purpose of separating sheets from the fixing member.
An example of using such a separation member is found in a fixing device having been disclosed. This means is provided with a gas discharge means made up of a separation guide plate whose one side approaches the surface of the fixing member and which is arranged lying in the circumferentially moving direction of the fixing member, and a plurality of nozzles for discharging pulse-like compressed air from the area sandwiched in-between the fixing member and the surface of the separation guide plate facing the same, toward the gap between the fixing member and one side of the separation guide plate approaching the same, a plurality of the aforementioned nozzles being arranged perpendicular to the circumferentially moving direction of the fixing member (refer to Unexamined Japanese Patent Application Publication No. 2005-202043).
Unexamined Japanese Patent Application Publication No. 2005-202043 uses the discharge of compressed air by the gas discharge means to separate the leading edge of the sheet, and uses a separation guide plate to separate the subsequent portion up to the trailing edge.
However, the separation failure of the sheet having passed through the nip section and winding of the sheet on the surface of the fixing member such as a fixing roller or fixing belt occur not only to the leading edge of the sheet, but also to the portion subsequent to the leading edge. Further, even when the leading edge of the sheet is pulled out by the sheet ejection roller, uneven separation remains on the wound portion. Thus, when air is to be blown to the outlet side of the nip section to separate the sheet from the fixing roller and others, air is preferably blown on a continuous basis.
However, after having separated the sheet from the fixing belt or fixing roller, the continuously blown air is repelled from the nip section to create a turbulent flow such as an eddy. The turbulent flow may cause the sheet to flap and may disturb conveyance of sheets. Particularly the thin sheet of smaller rigidity and smaller basis weight is subjected to such a turbulent flow, with the result that a conveyance trouble such as a jam may be caused. This adverse effect will be increased if the air flow is increased to improve the separation performance.
In view of the problems described above, it is an object of the present invention to provide a fixing device capable of minimizing flapping of the sheet caused by the air blown to separate the sheet from a fixing member and ensuring the stable sheet conveyance thereby.